The present invention relates to rotatable valve assemblies and more particularly, but not by way of limitation, to a rotatable valve assembly which may be used as a pressure relieving device.
The use of various sorts of rupture disks and pressure relief valves to prevent overpressure of a fluid pressure containment structure are known. For example, U.S. Pat. No. 3,472,284 (Hosek) discloses a pressure seal in which disks 26, 32 rotate about offset shaft 24. A diaphragm 14 is clamped between the disks and housing to seal the assembly and the diaphragm is sheared by the rotation of the disks when a desired fluid pressure is present. In order to reseal the valve, the housing members 10, 12 must be opened and the valve taken out of service.
U.S. Pat. No. 3,039,482 (Goldberg) discloses a butterfly-type valve in which the longitudinal axis of the inlet and the longitudinal axis of the outlet are offset so that when the valve is forcibly oscillated and the hermetic sealing sheet 13 is sheared, the sealing ring 18 on the lower edge of the valve 16 will safely clear the rough and sharp sheared edge of sheet 13. The inlet and outlet body sections 11, 12 must be taken out of service and opened in order to reseal the valve after a pressure relieving event.
U.S. Pat. No. 2,304,491 (Allen) and U.S. Pat. No. 3,603,333 (Anderson) disclose traditional right-angle relief valves in which a shear pin is used to hold the valve in a closed, sealed position until a preselected pressure is experienced in the inlet to the valve. Upon overpressure, the valve shears the shear pin and is moved to an open, pressure relieving position. Since both of these valves use a shear pin, the tolerances between the member holding the shear pin stationary and the moving member which shears the shear pin are critical to provide a predictable relief pressure. For example, referring to the Allen patent, the diameter of the stem 8 and the internal diameter of the bushing 9 through which the pin 13 passes must be carefully matched to achieve an accurately predictable shear pressure. As the space or gap between the sides of the stem 8 and the internal diameter of the bushing 9 increases, the force required to shear the pin 13 diminishes rapidly. This requires precision matching and matching of the componentry that is difficult and expensive to repeatably produce and to maintain. Other shortcomings of the right-angle relief valves include the increased body size and the flow restriction created by the right angle turn in the flow passageway, as well as the tendency of the valve to flutter or chatter on the seat during pressure fluctuations because the valve must resist the entire fluid pressure exerted at the inlet, i.e., the shear pin and spring must directly resist the full force exerted on the face of the valve by the inlet pressure and directly absorb all pressure fluctuations.
It is known to use rupture disks, such as reverse buckling rupture disks, to replace right-angle relief valves. Rupture disks have a lower cost per unit of capacity when compared to right-angle relief valves. However, rupture disks must be taken out of service to restore their pressure containing capability after a pressure relieving rupture. This requires either a redundancy (the use of multiple rupture disks in parallel), shutting down the system, or risking exposing the system to overpressure while the rupture disk is being replaced.
It is known to use rotatable valve assemblies, such as butterfly valves, ball valves, plug valves, and the like, to control or relieve fluid pressure in fluid containment systems. However, the prior rotatable valve assemblies known to the applicants typically require that the valve disk, ball, or plug, wipe or drag across a high friction, tight fitting seal made of elastomer or Teflon.RTM.. Therefore, a high torque is required to open the prior valve assemblies, and this torque increases with time while the valve is in a closed position.
Therefore, there is a need for a valve assembly which will provide fluid pressure relief at an accurately predictable relief pressure and which may be returned to its pressure containing state after a pressure relieving event without taking the apparatus out of service. There is also a need for a rotatable valve assembly which will reduce the frictional forces which must be overcome to rotate the valve between the open and closed position and in which the torque required to initiate rotation of the valve does not increase as the valve is left in a closed position for extended periods of time.